Recently, several reports have suggested that highly activated nN0S is partially responsible for the pathological effects of neurodegenerative diseases (Huang et al, Science 1994265:1883-1885: Zhang et al, Brain Res. 1994:654:85-95). S-methyl-L-thiocitrulline (MTICU), a potent and selective nN0S inhibitor (Ki=l.2 nM). has been reported to reduce the infarction size in a rat model of focal cerebral ischemia (Nagafuji et al, NeuroReport l995;6:1541-l545). MTICU is therefore a potential target for radiolabeling to probe nNOS in vivo. (C-11]MTICU was synthesized as published (Zhang et al, J. Label Compd. Radiopharm. 1995:37:240-242) with minor modification. Two mg of a-N-Boc-L-thiocitrulline t-butyl ester was alkylated with (C-11]methyl iodide at ll0 degrees C in acetone for 6 minutes. The solvent was evaporated under nitrogen before hydrolysis in 200 uL TFA at ll0 degrees C for 8 minutes. After HPLC purification, the final product was obtained within 50 minutes (Yield=9.1-12.5%. S.A.=22.2 GBq/mmol at E0S). HPLC analysis of blood samples. 30 minutes post-injection. indicated that (C-1]MTICU remained 64% intact in vivo, and 95% intact in vitro. The octanol/water partition coefficient (Log P=l.08+0.08 (n=6) and the biodistribution in female Sprague-Dawley rats were measured. Brain (0.11% 'ID/g) to blood (0.20% ID/g) ratio was 1:2 at 30 minutes post-injection. Total uptake in the brain was 0.19% ID/organ. Uptake of the tracer in the cerebellum, where nNOS distribution is most concentrated, was 20% higher than in both the cortex and the brain stem (p<0.05). (C-ll]MTICU uptake was also studied by co-administration of l mg/kg MTICU. The uptake in the cerebellum and the cortex were reduced by 22%, but the activity remained constant in the brain stem. (C-ll]MTICU was administered to a male baboon and brain images were obtained using a Siemens ECAT EXACT scanner. The (O-15]water and [O-15]carbon monoxide images were also obtained concurrently to detenmine the brain position and blood volume. Brain uptake of(C-ll]MTICU was stable ftom 5 minutes, remaining at 0.4% ID/organ, corrected for blood volume, up to one hour. Uniform distribution was observed in the brain of the normal animal. In conclusion, [C-11]MTICU is a tracer that has potential to be useful for the determination of nN05 levels in vivo.